1. Field of the Invention
The present invention relates to a pulse width modulation (PWM) circuit for controlling a duty factor of an output pulse thereof in response to an amplitude of an analog input signal, and in particular to a pulse width modulation circuit suitable for use to amplify the power of audio signals.
2. Discussion of the Background
A pulse width modulation (PWM) circuit is generally composed of an input terminal for receiving signals to be pulse width modulated, an integrating circuit, the input side thereof being connected to the input terminal, a pulse width modulated wave output terminal, and an output terminal for outputting a pulse modulated signal. The PWM circuit also includes a comparing circuit having a hysteresis characteristic for comparing an input signal with a reference signal applied to a reference input terminal to output the comparison result to the output terminal, and a feedback circuit for feeding back the pulse width modulated wave obtained at a pulse width modulated wave output terminal to the input terminal of the integrating circuit. Under the condition that no input signal is applied to the pulse width modulated wave input terminal, the following monotonous oscillating operation is repeated: a potential change of the integrating circuit (across a capacity) due to current flowing through the feedback circuit is acquired by the comparing circuit, and the direction of the current flowing through the feedback circuit is switched according to the polarity of the output of the comparing circuit. The pulse generated by this oscillation becomes a carrier. Here, when an analog input signal is applied to the pulse width modulated wave input terminal, since the potential change rate of the integrating circuit is subjected to the influence of the input signal amplitude, the polarity inversion time changes at the output of the comparing circuit. In other words, since the duty factor of the outputted pulse signal changes, it is possible to implement the pulse modulation according to the amplitude of the analog input signal.
In addition to the carrier self-oscillating type modulation circuit as described above, there exists another type modulation circuit in which a separate oscillator circuit for outputting a carrier pulse signal is connected to the input terminal of the integrating circuit.
In the conventional pulse width modulation circuit, however, although there exists an advantage in that it is possible to obtain a higher S/N ratio, i.e. a less erroneous operation of the comparing circuit, by increasing the amplitude of the signal outputted by the integration signal output terminal, there exists problems in that the dynamic range thereof is narrowed. Further this conventional circuit is not suitable to reduce the supply voltage of an LSI. This circuit is disadvantageous for high speed operation. Additionally, when the amplitude of the signal developed at the integration signal output terminal is reduced, although it is possible to secure a wide dynamic range and the circuit is suitable to allow reduction of the supply voltage of an LSI, there exists a problem in that it is difficult to secure a high S/N ratio. This tends to cause erroneous operation due to noise. Further, when the amplitude of the signal of the integration signal output terminal is small, that is, when the width of the hysteresis is narrow, there exists a problem in that the comparing circuit is easily operated erroneously due to noise.
Further, in the conventional pulse width modulation circuit, since the output level of the integrating circuit fluctuates according to the level of a dither signal, a problem arises in that the S/N ratio and the dynamic range are both low.
In addition, in the conventional pulse width modulation circuit, there exists another problem in that the components of the fundamental and higher harmonics of the carrier pulse signal are radiated unnecessarily, with the result that this exerts a harmful influence upon a wireless apparatus.
In order to eliminate the unnecessary radiation, it has been necessary to cover a printed circuit board or a whole apparatus. However, this countermeasure causes increase of size and/or cost of the apparatus by incorporating shielding plates made of a metal material.
To overcome the above-mentioned problems, a two-status modulation technique has been disclosed in European Patent Application No. 85303763.8 (Laid-Open No. 0184280A1), in which the width of the hysteresis of the integrating circuit can be changed. In this method, however, there still exists a drawback in that an erroneous operation easily occurs.